First, it would be good to know; is this a liner pond or preformed? From one pic, it seems like a preform. When you listed the depth at 1', that's alarming in that the pond must freeze up pretty deep in Pa. A good person to also ask would be
@poconojoe as he's a neighbor of yours. Typical ponds are more like 18" to a more workable 3-4'. You'll have to watch that it doesn't freeze all the way down. Since you say the previous owner didn't do anything, he probably didn't tell you if the pond ever froze solid. Goldfish are tough and can survive being frozen (not ideal and certainly not 100% proof of survivability).
First thing I'd do is get yourself a kiddie pool large enough to hold enough water and get the goldfish out. This will make taking all the muck out a lot easier. Normally we advise to just run a net slowly along the bottom and fish it out but in your case, I'd just remove the fish and then clean the muck. Seems a lot easier and quicker. I'd fill the kiddie pool with the top layer of your pond water--you want this as the water has cycled.
Now realize, the sides and bottom of your pond have bacteria on them that is valuable--it'll help re-start the nitrogen cycle which may keep you from getting an algae bloom. If you can lift the fish, siphon out the water, clean the muck and then refill in a timely fashion, you can hopefully keep the situation from magnifying. Put the fish in buckets and float them in the new water (which also has the kiddie pool water put back in) and acclimate the fish relative to temp. If you're filling using city water, don't forget the dechlorinator as fish can't handle the chlorine/chloramine that's in municipal water). I'd float for 30 minutes or so. If you're lucky, your new water will be close in temp to the kiddie pool water and you'll not stress them much. Again, goldfish are tough.
With all that said;, ideally you want more depth and can consider replacing the preform with a liner pond in which you'd dig deeper to a better depth. That's one option and the one I'd consider as 12" is just too shallow, imo. It'll work, but there's lots of limitations.
Now, once that is initialized, we can talk filtration. Without even knowing what you have, and it won't matter unless you already have my suggestion, consider creating a bog filter. This can be done before you even do the cleaning described above. A bog, more appropriately called 'upflow wetland filtration', is a structure (or side pond, if you want to dig) that houses at least 12" of gravel (pea gravel; round edged stone) and has pipe buried on the bottom whose supply comes from a pump located in your pond. You could also get an external; just more plumbing. I like the submersibles.
This piping from the pond via the pump sends water to the bottom of your bog where it'll rise up through the gravel and pour back into the pond. This creates a waterfall for aeration (something you want to battle algae and help the fish thrive) and allows all the bacteria colonizing your new bog pea stone to break down the bad elements in your pond water. You'll put plants on top and they take out the third bad element; nitrates. Get yourself some floating plants to place in the pond to go along with what looks like your water lily and you'll be set.
With what looks like your waterfall, you'll have a pump that supplies water to it; it may or many not be strong enough to Y off of so you get that bog feed. If not, get another pump or buy a larger one that will feed both.
With all that said, once you clear the pond of the muck, change out the water, provide excellent filtration (once built, you'll not have to do anything again except thin plants now and then in the top of your bog) as it'll give you water clear enough that now the predators can more easily see your fish. You'll consider a net now as this is the only real way to protect them. Or not; it's your pond but once a heron finds it, the fish won't last.
Okay, lots of info. Any questions, fire away as there's lots of good knowledgeable people here to help. And don't let the above scare you; it seems like a lot but you can knock most of this out in a weekend and you'll reap many years of benefits. Do a search re 'bogs' here and you'll find a lot more info to help you learn about such filtration and if such is something you're interested in.
Welcome to GPF!
pondinformer.com
but if you understand this, you can get a pond that is balanced and your fish will be happier.
